NEW YORK (Reuters Health) - Babies diagnosed with jaundice may be more likely to later receive a diagnosis of autism, suggests a large new study.

However, the Danish researchers caution that many questions remain unanswered, making it too early to say for sure if there is a true cause-and-effect relationship between the conditions.

Environmental exposures prior to, during and shortly after birth are emerging as important risk factors for the development of autism, in addition to genetic factors, Hannah Gardener of the University of Miami Miller School of Medicine, who was not involved in the study, told Reuters Health in an e-mail.

Jaundice is a common condition among newborns that results when the yellow pigment found in bile, called bilirubin, accumulates faster than the immature liver can process it. More than half of babies born full-term have some of the characteristic yellowing of the skin and eyes, but it usually resolves itself and is rarely harmful.

Autism, which causes problems with social and communications skills, affects approximately one in every 110 U.S. children, according to the Centers for Disease Control and Prevention.

In a prior study, Dr. Rikke Damkjaer Maimburg of Aarhus University found that children diagnosed with autism were twice as likely to have been admitted to the neonatal care unit as newborns, most commonly for jaundice. Yet she knew that earlier research into a link between jaundice and autism had yielded conflicting results.

So Maimburg and her colleagues decided to conduct a larger and more rigorous study than those done in the past. They retrieved detailed information from national registries on nearly all babies born in Denmark between 1994 and 2004.

Of the 733,826 children born during that period, 35,766 were born with jaundice, 1,721 were later diagnosed with a psychological disorder of some kind and 577 developed autism.

After adjusting for other factors such as birth weight and mother's smoking status, the team determined that children born full-term with jaundice had a 56 percent greater chance of developing an autistic spectrum disorder later on than those without jaundice, report the researchers in the journal Pediatrics.

These same children were also at a significantly greater risk of developing a range of other developmental psychological problems, such as learning or speech disorders.

Looking more closely at the data, the researchers found that first-born children and babies born earlier than the 37th week of pregnancy seemed to be protected from the apparent effects of jaundice. Those born during spring and summer months also seemed to be unaffected.

They speculate that whatever damage the jaundice may be inflicting occurs during the last few weeks of gestation, and that seasonal environmental factors after birth may lessen or worsen the problem.

"The best guess as to how jaundice causes changes in psychological development is that bilirubin crosses the blood-brain barrier and destroys brain cells, as we know it does in cerebral palsy," Maimburg told Reuters Health in an e-mail.

Increased brain development in the last weeks of gestation as well as a higher rate of infections and less sunlight (which can help in the breakdown of bilirubin) during winter months might explain some of the differences identified, note the researchers. Antibodies accumulated during previous pregnancies could also contribute to the greater effect seen in children who were not first-borns, they add.

The fact that most previous studies failed to show an association between jaundice and autism may be due to their not having broken down the data by these other potential factors, said Gardener.

The new study has its own limitations, she added, including a jaundice rate far lower than what is estimated in the general population. It is likely that only the most severe cases were captured because the researchers drew from diagnostic codes in hospital records and mild cases may not all be included there.

"Jaundice is a common exposure during the neonatal period and should not be ignored," said Gardener. "Identification and treatment of jaundice is important for both pre-term and term neonates."

Maimburg suggested that post-birth classes with lectures on how parents can observe their newborns and recognize when jaundice requires attention could help to minimize the potential risk. But she emphasized that parents shouldn't be alarmed if they do spot a yellow tint to their newborn's skin, as most will develop at least a mild case of jaundice during the first days of life.